Surface coated smokeless propellant charge for caseless ammunition having heat resistant properties

ABSTRACT

A propellant charge for caseless ammunition is provided comprising a base propellant charge and a heat and combustible combustible surface coating bonded to the exterior surface of the base charge. This surface coating functions to prevent undesired autoignition of the base charge. The surface coating is an intimate admixture of a crosslinked polyacrylic rubber binder and an organic oxidizer.

Waited States Fatent [191 Eraak et a1.

[ 1 Jan. 30, 1973 [54] SURFACE COATED SMOKELESS PROPELLANT CHARGE FOR CASELESS AMMUNITION HAVING HEAT RESISTANT PROPERTIES [7 5 Inventors: Edmund C. Braak, Landing, NJ. 07850; Kenneth P. McCarty,

A Sa1t Lake City, Utah 84106 [73] Assignee: Hercules Incorporated, Wilmington, Del.

[22] Filed: July 7,1970

21 1 Appl. No.: 52,904

[52] US. Cl. ..149/12, 149/19, 149/96, 149/98, 149/105 [51] Int. Cl. ..C06b 19/02 [58] Field ofSearch ..149/l2, 19,92, 105,96, 98, 149/106; 102/99, 103

[56] References Cited UNITED STATES PATENTS SURFACE TEMPERATURE (Fl CONTROL PROPEL 6/1964 Wright .[1'4'9/19l0 3,348,986 10/1967 Sauer "149/12 X 3,354,010 11/1967 Hopper et al ..149/2 3,386,868 6/1968 Glmler et a1 ..l49/19 X 3,440,115 4/1969 Falterman et a1 149/92 X 3,441,549 4/1969 Gardiner et al ..i49/19 X 3,480,490 11/1969 Finger et a1 ..149/19'X 3,507,719 4/1970 i-Iodgson ..l49/19X Primary Examiner-Carl D. Quarforth Assistant Examiner-P. A. Nelson AttorneyMichael B. Keehan [57] ABSTRACT A propellant charge for caseless ammunition is provided comprising a base propellant charge and a heat and combustible combustible surface coating bonded to the exterior surface of the base charge. This surface coating functions to prevent undesired autoignition of the base charge. The surface coating is an intimate admixture of a crosslinked polyacrylic rubber binder and an organic oxidizer.

7 Claims, 1 Drawing Figure TIME TO AUTOIGNITION (SECJ PATENTEDJMI 30 I973 m m 5425 mmmzzuzt. GZF O0 mn- OOH OOO

EDMUND .c. BRAAK KENNETH P McCARTY INVENTORS ATTORNEY SURFACE COATED SMOKELESS PROPELLANT CHARGE FOR CASELESS AMMUNITION HAVING HEAT RESISTANT PROPERTIES This invention relates to a propellant charge for a round of caseless ammunition exhibiting improved heat resistant properties. More particularly this invention relates to a propellant charge for caseless ammunition comprised of a base charge and a heat resistant propellant composition bonded to the exterior surface of the base charge.

In case-type ammunition a granular propellant charge is contained within a brass case. The brass case functions to hold the propellant charge and projectile and to delay heat transfer to the granular propellant charge from a gun chamber. In contrast, the propellant charge of a round of caseless ammunition comes in direct contact with the gun chamber in which it is to be fired. In a rapid fire gun system, the heat build-up within the gun chamber is substantial. The high temperature of a gun chamber can cause premature ignition of the propellant charge of a caseless round which is a safety and operational hazard.

Accordingly, it is desirable to provide a propellant charge for a round of caseless ammunition which will have a heat resistant and combustible exterior surface which will function to prevent autoignition of the propellant due to gun chamber temperature and yet will be completely consumed within the chamber of the gun as a result of firing the caseless round.

In accordance with this invention a propellant charge for a round of caseless ammunition is provided having in combination, a base charge comprising consolidated granules of smokeless powder, said base charge having an exterior surface, and a heat resistant propellant composition bonded to the exterior surface of the base charge said heat resistant propellant composition comprising by weight from about to about 30 percent of a crosslinked polyacrylic rubber binder consisting essentially of about 97 percent ethyl acrylate and about 3 percent of a material of the group consisting of combined N-methylol acrylamidezacrylamide and combined N-methylol acrylamide:methacrylamide in intimate admixture with from about 70 to about 90 percent of particulate organic oxidizer of the group consisting of cyclotetramethylene tetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX).

The base charge of this invention is prepared from consolidated granules of smokeless powder. The smokeless powder can be of the single base, double base or triple base type. Consolidation of the propellant granules can be done by any suitable process such as by molding with or without the use of a binder material. A base charge prepared from molded granules of smokeless powder has a series of interstitial passages or pores throughout the charge. The surface of a molded base charge is irregular having a plurality of pores distributed about the exterior surface of the charge.

The heat resistant propellant composition that is bonded to the exterior surface of the base charge comprises a binder and an oxidizer. The binder is a crosslinked polyacrylic rubber employed in an amount of from about 10 percent to about 30 percent and preferably from about 10 percent to about percent by weight of the composition. The oxidizers employed are cyclotetramethylenetetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX) which are organic oxidizers which burn completely, are non-corrosive, and have no noticeable residue. These oxidizers are used in an amount of from about percent to about 90 percent and preferably from about percent to about percent by weight of the composition. The oxidizers are incorporated in the composition in particulate form and generally have a particle size range of from about 2 microns to about 30 microns.

The heat resistant propellant compositions employed as a coating for the base charge in preparation of the propellant charge of this invention are prepared using conventional smokeless powder equipment and using a method of preparation as set forth in U.S. Pat. No. 3,152,027 where volatile solvent is utilized for desensitizing the doughy mixture of binder and particulate oxidizer. Illustrative heat resistant propellant compositions employed to coat the base charge of this invention are set forth in Table 1 below.

(3) Hycar 2121-X-66 is sold by the B. F.

Goodrich Co.

and is a polyacrylic rubber consisting of about 97% ethyl acrylate and of about 3% combined N-methyloloacrylamide:methacrylamide Additional data on these compositions is set forth in US. Pat. No. 3,386,868 to J. R. Gimler et al., reference to which is hereby made.

The propellant charge of this invention is prepared by applying the heat resistant propellant composition to a base charge by any suitable means such as spraying or dipping. One particularly suitable means of applying this coating composition to the base charge is to prepare a solution--suspension of the coating composition and to dip the base charge into the solutionsuspension of the coating composition. The base charge is withdrawn from the solution-suspension of coating composition at a controlled rate to vary the coating thickness. More than one dip coating step can be employed as necessary to obtain the desired coating thickness. After dip coating the resultant propellant charge can be dried in about 30 minutes at ambient temperature. The coating composition is bonded to the base charge.

A solution-suspension of the heat resistant propellant composition is prepared by admixing a diluent or suspending agent and the heat resistant propellant composition for a time sufficient to provide the solution-suspension of the composition. Diluents or suspending agents employed in this invention are solvents for the binder employed. Illustrative diluents which can be employed include acetone, methyl ethyl ketone, methyl isobutyl ketone, ethyl ether, ethyl acetate, benzene, toluene, the xylenes, trichloroethylene, ethanol, and mixtures thereof. The solution-suspension of the heat resistant coating composition can contain up to about 50 percent and preferably from about 20 percent to about 40 percent by weight of heat resistant propellant composition based on the weight of the solution-suspension composition. A particularly suitable diluent is a mixture comprising about 95 percent by weight of toluene and about 5 percent by weight of methyl ethyl ketone.

The heat resistant and combustible material which is bonded to the surface of the base charge can be cured if desired. Curing of the coating composition is affected by heating the composition at a temperature of from about 130-200 F. for about 16 hours. Higher curing temperatures can be employed and will increase the crosslink density of the coating composition. Upper temperature limits of curing are principally determined by the effect of high temperatures on the base charge. The heat resistant propellant composition can optionally contain a crosslinking rate accelerator such as salicylic acid, benzoic acid, or phthalic anhydride. Elimination of the crosslinking rate accelerator neces sitates a longer curing cycle for the propellant composition, but the resulting propellant is essentially the same. From about 0.1 to about 0.5 percent by weight of crosslinking rate accelerator has been found satisfactory. The heat resistant propellant composition can also contain from about 0 percent to about 1 by weight of wax such as paraffin wax.

An alternative method of preparing the propellant charge of this invention is to first prepare a thin film of the heat resistant propellant composition by casting a solution-suspension of the heat resistant propellant composition onto a surface, and drying and curing the composition. The propellant composition, now in the form ofa thin film can be adhesively secured to the surface of the base charge.

When preparing the propellant charge of this invention, it is desirable to fill the surface pores of the base charge with a pore filling material. Pore filling materials must be combustible, compatible with both the base charge and the coating composition and should readily adhere to the surface of the base charge. One suitable pore filling material for a base charge is a composition comprising about equal parts by weight of methyl abietate, sold commercially under the trade name Abalyn;" oxamide; cyclotetramethylenetetranitramine (HMX); and nitrocellulose containing 1 1.8- 12.2 percent by weight of nitrogen available commercially as RS" nitrocellulose, '/4 second grade from Hercules Incorporated.

The pore filling material having this composition can be prepared by dissolving Abalyn in a mixture of mineral spirits and isopropyl alcohol. Nitrocellulose is added to the solution and the resulting admixture is ball milled to reduce the average particle size of the nitrocellulose to about p. Cyclotetramethylenetetranitramine (HMX) Class B and oxamide are admixed with the mixture of Abalyn" and nitrocellulose. The resulting material is a pore filling material having the consistency of a creamy paste. This paste can be employed to fill the surface pores of the base charge by any method, such as by wiping or knife coating. The pore filled base charge can be surface coated with the propellant composition described herein immediately following the application of the pore filler to the base charge. One advantage to a pore filled base charge is that the heat resistant propellant composition bonded to a pore filled base charge forms a continuous surface covering which encapsulates the base charge. The resulting propellant charge has increased moisture resistance because of the moisture resistant properties of the propellant composition coating the base charge.

The following examples more fully illustrate this invention. In the examples which follow parts and percentages are by weight unless specified otherwise.

Example 1 which follows illustrates a process for preparing a solution-suspension of the heat resistant combustible composition employed in this invention.

EXAMPLE 1 About 40 parts of a heat resistant propellant composition comprising 84 percent cyclotetramethylenetetranitramine, 15 .8 percent Hycar 2lX-66, and 0.16 percent salicylic acid is admixed with 60 parts of toluene. The resulting admixture is a solution-suspension of the heat resistant composition employed for coating the base charge when preparing the propellant charge of this invention.

Example 2 which follows illustrates the protection from premature ignition provided by the surface coating of this invention.

EXAMPLE 2 The solution-suspension of propellant prepared in Example 1 is knife-coated on glass at several wet thicknesses. The solvent from the propellant solutionsuspension is evaporated from the propellant and the propellant is cured. The resulting propellant film is soaked in water and peeled from the glass. Circular discs are cut from the propellant film. These propellant discs are secured to the exterior surface of a consolidated charge of smokeless powder of the doublebase type employing an adhesive comprising a thin smear of the solution-suspension of propellant prepared in Example 1. Adhesion of the dried propellant film to the base charge is excellent. The thickness of the coating applied to the base charge is obtained by difference in measurement between the thickness of the uncoated propellant and the thickness of the coated propellant.

The propellant samples coated with the heat resistant coating composition as heretofore described contain about 1.5 grams of molded granules of smokeless powder and are prepared in the form of small cylinders one half inch in diameter by three eights inch in length. Autoignition tests on the propellants are conducted as follows. A propellant sample is secured to the base of an inverted metal basket open at one end. A one eighth inch air gap exists between the propellant sample and the inner basket walls. The propellant sample protrudes between about one sixteenth and one eighth inch below the open end of the basket providing a propellant contact surface. To conduct an autoignition test the basket containing the propellant sample is remotely lowered to a hot plate which has a surface which is closely temperature controlled. The propellant cylinder is held in contact with the hot metal surface by the weight of the inverted metal basket, the pressure on said propellant cylinder being approximately 0.876lbs./in. There is unrestricted vertical movement of the propellant during the autoignition test. Lateral movement is restricted, however, by a loose fitting steel guide about the propellant basket. The principal function of contact pressure is to improve heat transfer between the surface of the hot plate and the propellant charge. Time to autoignition (Tai) is measured by starting a clock as the sample is contacted with the hot metal surface and stopping the clock at the first sign of flame or burning.

Propellant samples coated with the temperature resistant propellant composition at coating thicknesses of about 3, 5, 7, and 9 mils are subjected to autoignition tests following the procedure heretofore described. Results of conducting these tests are shown in the drawing. A control sample employing an identical base charge which was not coated with the temperature resistant propellant composition is subjected to the same autoignition tests described. Results of this test are also plotted on the drawing. The curves plotted on the drawing show that the time to autoignition for coated propellant charges of this invention is substantially increased compared to an uncoated base charge. Time to autoignition increases with the coating thickness at a given surface temperature. This increase in time to autoignition is attributed to the increased mass heat-absorbing capacity of the thicker coatings.

What we claim and desire to protect by Letters Patent is:

l. A propellant charge for a round of caseless ammunition having in combination:

a. a base charge comprising consolidated granules of smokeless powder, said base charge having an exterior surface with a plurality of pores throughout said exterior surface, and

b. a heat resistant propellant composition bonded to and providing a covering over the exterior surface of the base charge, said heat resistant propellant composition comprising by weight from about 10 percent to about 30 percent of a crosslinked polyacrylic rubber binder consisting essentially of about 97 percent ethyl acrylate and about 3 percent of a material of the group consisting of combined N-methylol acrylamidezacrylamide and combined N-methylol acrylamide:methacrylamide in intimate admixture with from about to about percent of particulate organic oxidizer of the group consisting of cyclotetramethylenetetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX).

2. The propellant charge of claim 1 in which the heat resistant propellant composition bonded to the base charge has a thickness of from about 3 mils to about 9 mils.

3. The propellant charge of claim 1 in which the consolidated base charge is prepared from molded granules of double-base type smokeless powder.

4. The propellant charge of claim 3 wherein the heat resistant propellant composition comprises by weight from about 10 percent to about 30 percent of a crosslinked polyacry to rubber binder consisting essentially of about 97 percent ethyl acrylate and about 3 percent of N-methylol acrylamide:methacrylamide in intimate admixture with from about 70 to about 90 percent of particulate cyclotetramethylenetetranitramine (HMX).

5. The propellant charge of claim 3 wherein the heat resistant propellant composition comprises by weight from about 10 percent to about 30 percent of a crosslinked polyacrylic rubber binder consisting essentially of 97 percent ethyl acrylate and about 3 percent N- methylol acrylamidezacrylamide in intimate admixture with from about 70 percent to about 90 percent of cyclotetramethylenetetranitramine (HMX).

6. The propellant charge of claim 1 in which the pores at the exterior surface of the consolidated base charge are filled with a combustible pore filling material and the heat resistant propellant composition is bonded to the exterior surface of the pore filled base charge providing a continuous surface covering over the base charge.

7. The propellant charge of claim 6 wherein the heat resistant propellant composition bonded to the exterior surface of the pore filled base charge has a thickness of from about 3 mils to about 9 mils. 

1. A propellant charge for a round of caseless ammunition having in combination: a. a base charge comprising consolidated granules of smokeless powder, said base charge having an exterior surface with a plurality of pores throughout said exterior surface, and b. a heat resistant propellant composition bonded to and providing a covering over the exterior surface of the base charge, said heat resistant propellant composition comprising by weight from about 10 percent to about 30 percent of a crosslinked polyacrylic rubber binder consisting essentially of about 97 percent ethyl acrylate and about 3 percent of a material of the group consisting of combined N-methylol acrylamide:acrylamide and combined N-methylol acrylamide: methacrylamide in intimate admixture with from about 70 to about 90 percent of particulate organic oxidizer of the group consisting of cyclotetramethylenetetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX).
 2. The propellant charge of claim 1 in which the heat resistant propellant composition bonded to the base charge has a thickness of from about 3 mils to about 9 mils.
 3. The propellant charge of claim 1 in which the consolidated base charge is prepared from molded granules of double-base type smokeless powder.
 4. The propellant charge of claim 3 wherein the heat resistant propellant composition comprises by weight from about 10 percent to about 30 percent of a crosslinked polyacrylic rubber binder consisting essentially of about 97 percent ethyl acrylate and about 3 percent of N-methylol acrylamide:methacrylamide in intimate admixture with from about 70 to about 90 percent of particulate cyclotetramethylenetetranitramine (HMX).
 5. The propellant charge of claim 3 wherein the heat resistant propellant composition comprises by weight from about 10 percent to about 30 percent of a crosslinked polyacrylic rubber binder consisting essentially of 97 percent ethyl acrylate and about 3 percent N-methylol acrylamide:acrylamide in intimate admixture with from about 70 percent to about 90 percent of cyclotetramethylenetetranitramine (HMX).
 6. The propellant charge of claim 1 in which the pores at the exterior surface of the consolidated base charge are filled with a combustible pore filling material and the heat resistant propellant composition is bonded to the exterior surface of the pore filled base charge providing a continuous surface covering over the base charge. 